Instead of using separate wells for steam injection and oil production, a single well can be used for the injection of steam and the production of oil. In such operation the injection of steam and the production of oil occur in a cyclic mode generally referred to as Cyclic Steam Simulation (CSS) process. In the CSS process, the well is shut in and steam is injected through the well into the oil-bearing formation to lower the viscosity of the oil. During a next stage, oil is produced from the formation through the same well. In order that the steam is injected substantially uniformly along the portion of the well penetrating the reservoir zone, i.e. without a concentration of injected steam at one location at the cost of another location, the steam is generally pumped through spaced outlet ports having a relatively small diameter, generally referred to as Limited Entry Perforations (LEP). This is done to ensure that the steam exits the outlet ports at a velocity approaching sonic velocity and is therefore choked or throttled. The size of the outlet ports typically is of the order of 0.5-1.0 inch.
U.S. Pat. No. 6,158,510 suggests a wellbore liner for CSS including a base pipe provided with a plurality of LEP ports spaced in longitudinal direction and circumferential direction of the liner. The liner is provided with several sandscreens spaced along the liner, each sandscreen extending around the base pipe at short radial distance therefrom. During each steam injection cycle, the well is shut in and steam is injected into the rock formation via the LEP ports. The steam flows through the LEP ports at sub-critical velocity so that the flow rate of steam in the LEP ports is independent from pressure variations downstream the ports, thus ensuring a uniform outflow of steam along the liner. After a period of steam injection, a production cycle is started whereby oil from the surrounding rock formation flows via the LEP ports into the liner and from there to a production facility at surface.
It is a drawback of the known system that, during the production cycle, the volumetric flow rate of oil through the LEP ports is relatively low. The amount of oil produced from the well in a given period of time is therefore also low.
U.S. Pat. No. 5,865,249. discloses a system configured to flush debris from the bottom of a wellbore by injecting water via a water injection conduit into the plugged zone and inducing the debris to flow up through the wellbore through the production conduit.